Syncope is a frequent symptom in patients with hypertrophic cardiomyopathy, and may be caused by ventricular tachycardia in many patients, due to a fall in cardiac output. Supraventricular tachycardia is generally well tolerated by patients with normal left ventricles, and rarely results in syncope. To examine the effect of supraventricular tachycardia in hypertrophic cardiomyopathy, rapid atrial pacing at a heart rate of 150 was performed with measurement of mean blood pressure in 25 patients with hypertrophic cardiomyopathy and 21 patients with normal left ventricles. Coronary flow was estimated in the great cardiac vein in 12 patients with hypertrophic cardiomyopathy and 8 patients with normal left ventricles, all with normal epicardial coronary arteries. In comparison to patients with normal left ventricles, patients with hypertrophic cardiomyopathy demonstrated a marked fall in systemic blood pressure during rapid atrial pacing. This may relate to a fall in preload due to shortened diastolic filling period and loss of coordinated atrial systole in a noncompliant ventricle operating on an ascending limb of the Starling curve, resulting in a fall in stroke work. The fall in blood pressure also resulted in a marked fall in coronary flow, which may cause myocardial ischemia and left ventricular dysfunction. Thus, supraventricular tachycardia, simulated by rapid atrial pacing, may account for presyncope and syncope in many patients with hypertrophic cardiomyopathy.